Measuring the number and energy of each particle of light or photon since the Big Bang could provide secrets about the nature and evolution of the Universe, such as how similar or different ancient galaxies were compared to the galaxies today.

The researchers made the most accurate measurement of the extragalactic background light (EBL) by using observations spanning wavelengths from radio waves to very energetic gamma rays. Directly measuring the EBL by collecting its photons with a telescope poses towering technical challenges. Earth sits in a very bright Solar System, which almost acts like light pollution when trying to make reliable EBL measurements.

Astrophysicists had to measure the EBL through measuring the attenuation of very high energy gamma rays from distant blazars, which are supermassive black holes in the centers of galaxies with brilliant jets pointed towards Earth. Not all the high-energy gamma rays emitted by a blazar make it all the way across billions of light-years to Earth, because some of it strikes an EBL photon along the way. When this happens, both are annihilated and produce two different particles: an electron and a positron.

Measuring how much gamma rays of different energies are attenuated or weakened from blazars at different distances from Earth directly gives a measurement of how many EBL photons of different wavelengths exist along the line of sight from blazar to Earth over those distances.

NASA’s Fermi Gamma Ray Telescope first detected that gamma rays from distant blazars are attenuated more than gamma rays from nearby blazars. Now, the scientists have been able to measure the evolution of the EBL over the past five billion years.

The researchers had to compare the Fermi findings to the intensity of X-rays from the same blazars measured by X-ray satellites Chandra, Swift, Rossi X-ray Timing Explorer, and XMM-Newton and lower-energy radiation measured by other spacecraft. These measurements enabled the team to calculate the blazars’ original emitted, unattenuated gamma-ray brightness at different energies.

The authors compared their calculations of unattenuated gamma-ray flux at different energies with direct measurements from ground-based telescopes of the actual gamma-ray flux received at Earth from those same blazars. This allowed the team to quantify the evolution of the EBL out to about five billion years ago.

“Five billion years ago is the maximum distance we are able to probe with our current technology,” Alberto Dominguez, of the Department of Physics and Astronomy at the University of California, said in a statement. “Sure, there are blazars farther away, but we are not able to detect them because the high-energy gamma rays they are emitting are too attenuated by EBL when they get to us–so weakened that our instruments are not sensitive enough to detect them.”

Their measurement was the first statistically significant detection of the “Cosmic Gamma Ray Horizon” as a function of gamma-ray energy. The Cosmic Gamma Ray Horizon is defined as the distance at which one-third of the gamma-rays of a particular energy have been attenuated.

The latest result confirms that the kinds of galaxies observed today are responsible for most of the EBL over all time. It also sets limits on possible contributions from many galaxies too faint to have been included in the galaxy surveys.

Image Below: The attached figure illustrates how energetic gamma rays (dashed lines) from a distant blazar strike photons of extragalactic background light (wavy lines) and produce pairs of electrons and positrons. The energetic gamma rays that are not attenuated by this process strike the upper atmosphere, producing a cascade of charged particles which make a cone of ÄŒerenkov light that is detected by the array of imaging atmospheric ÄŒerenkov telescopes on the ground. Credit: Nina McCurdy and Joel R. Primack/UC-HiPACC